This invention relates to aqueous dispersions of S-EB-S block copolymer and the use of such aqueous dispersions in forming articles, preferably by dip forming.
Thin-walled, extensible articles such as gloves, condoms, and other products have long been made from natural rubber. In normal production, such articles are formed from natural rubber latex, a naturally occurring emulsion of rubber and water, with added stabilizing agents and vulcanizing chemicals. A form of the appropriate shape, previously coated with a coagulating solution in some cases, is dipped into the latex mixture once or several times to build up a layer of the desired thickness. The water is allowed to evaporate, leaving a solid rubber film. The film must be vulcanized to provide adequate mechanical and physical properties.
Natural rubber has many advantages in these applications, being strong and highly elastic and having good xe2x80x9ctactilityxe2x80x9d or feeling to the user. Natural rubber has several shortcomings, such as susceptibility to xe2x80x9cpinholesxe2x80x9d therethrough, rapid attack by ozone which causes scission cracking, and oxidative attack during storage which causes cracking and destroys the physical integrity of the product. Natural rubber is also not hypoallergenic due to the residual surfactants, vulcanizing agents, stabilizing agents, antioxidants, and/or protein materials in the rubber. Persons who are particularly susceptible to irritation or sensitization, or who use the rubber products for extended periods of time, may experience allergic reactions.
Various types of synthetic elastomeric polymer products have been developed for use in thin articles produced by dip forming. Synthetic rubber compositions may be dissolved in solvents to form a true solution, so that pinholes are much less likely to be present. Many available synthetic rubber compositions have various other shortcomings, including unacceptable tactility. While each such composition may meet some of the requirements, most do not have the required combination of strength, tactility, resistance to environmental damage, and hypoallergenicity required for many products such as examination and surgical gloves, condoms, and other medical products that are to come into contact with the human body.
An important advance in the art of synthetic elastomeric polymer products is described in U.S. Pat. Nos. 5,112,900 and 5,407,715. These patents disclose the preparation of specific styrene-ethylene/butylene-styrene (S-EB-S) block copolymer solutions and their use in the dip forming of articles. The resulting articles have excellent elastomeric properties for use in gloves, condoms, and other products. They exhibit low incidence of pinholes, good resistance to environmental damage such as oxidation and ozonation, and hypoallergenicity.
There is, however, always a need to further improve the manufacturability of articles made of such formulations and the process economics. The present invention fulfills this need, and further provides related advantages.
The present invention provides an S-EB-S aqueous dispersion and an approach for utilizing that aqueous dispersion in the preparation of thin elastomeric articles. The articles have the desirable characteristics of comparable articles made from S-EB-S solutions, including excellent elastomeric properties, low incidence of pinholes, good resistance to environmental damage such as oxidation and ozonation, and hypoallergenicity. The present approach is compatible with related technology such as the use of powders and powder-free techniques for improving the donnability of the articles. Additionally, the dispersion-based dip-forming manufacturing operation functions at greater rates for improved process economics, as compared with the prior approach of dip forming from S-EB-S solutions. Thicker layers or articles may be made in each dip-forming step. The manufacturing operation is also safer due to the absence of toxic solvents during the dip-forming process.
In accordance with the invention, an aqueous dispersion comprises a dispersion medium comprising a mixture of water and a surfactant, and a plurality of particles dispersed in the dispersion medium. Each particle comprises a mixture of an S-EB-S (styrene-ethylene/butylene-styrene) block copolymer, and an oil such as a mineral oil. Most preferably, the particles are of an average size of no greater than about 2 micrometers. The S-EB-S block copolymer, which may be formed of molecules of substantially the same molecular weight or mixtures of two or more molecular weights, preferably having weight average end block molecular weights of each of the end blocks of more than about 15,000 Daltons.
The aqueous dispersion may be used in a dip-forming method for manufacturing thin-walled articles. In accordance with this aspect of the invention, a method for the preparation of an elastomeric article comprises the steps of furnishing an aqueous dispersion of the type described, dipping a form into the aqueous dispersion and withdrawing the form from the aqueous dispersion, leaving a film of the dispersion on the form, and evaporating the water from the dispersion on the form and fusing the remaining polymeric material, leaving a coherent extensible film on the form. The aqueous dispersion used in the dip-forming method is substantially free of non-aqueous solvents, but trace amounts that may be present are not detrimental in the dip-forming process, and may, in some cases, be beneficial in forming a coherent film.
The aqueous dispersion may be used in conjunction with other processing techniques. For example, the aqueous dispersion may be used in the coagulant dipping process that is utilized for natural latex rubber compounds. It also may be used, for example, in spray coating or slush molding operations.
The resulting article has the desirable features associated with the S-EB-S block copolymer as described U.S. Pat. Nos. 5,112,900 and 5,407,715, whose disclosures are incorporated by reference. Additionally, the articles may be made much more quickly than possible with the solution-based approach described in these prior patents. In the manufacturing operation, the dispersion is normally made at a location which has apparatus for disposing of the potentially toxic solvent vapors evolved during preparation of the dispersion. The dip-forming operation may be performed elsewhere. Because there is no potentially toxic solvent evolved in the dip-forming operation, there is little risk of injury to workers on the dip-forming and drying line.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.